The proton affinities of seven matrix-assisted laser desorption/ionization matrices correlated with the formation of multiply charged ions

Abstract

The gas-phase proton affinities (PAs) of seven matrices commonly used for matrix-assisted laser desorption/ionization (MALDI) have been measured by the kinetic method. The results were, with the PA in parenthesis: α-cyano-4-hydroxycinnamic acid (841 kJ mol-1), caffeic acid (866 kJ mol-1), ferulic acid (879 kJ mol-1), sinapinic acid (887 kJ mol-1), 3-hydroxypicolinic acid (896 kJ mol-1), nicotinic acid (907 kJ mol-1), picolinic acid (907 kJ mol-1) and 2-(hydroxyphenylazo)benzoic acid (943 kJ mol-1). The analysis of MALDI spectra of insulin, ribonuclease A and myoglobin show, for insulin and myoglobin, a clear correlation between the matrix PA and the yield of multiply charged ions, with a low matrix PA associated with high yields of multiply charged analyte ions. Comparison with other experiments further suggests that matrices with low PA values induce more fragmentation of the analyte than matrices with high PA values. The results are in agreement with the hypothesis that gas-phase proton transfer reactions play a significant role in the formation of analyte ions under MALDI conditions.